1. Field of the Invention
The present invention relates to an enhanced arrangement for supplying power to a circuit board, and in particular, to an enhanced arrangement for supplying power to a backplane of a computer using a buss bar.
2. Background Information
Backplanes are wiring boards used, for example, in computers, and are typically provided with card slots or plugs for receiving various circuit boards, such as a processor card and a so-called memory riser card, attached to a common surface of the backplane by way of the card slots or the plugs. Further, it is typical to permanently attach a clock card, which is a further type of circuit board, and various electrical components, to the backplane. This assembly of the backplane, electrical components, and circuit boards may be positioned within an open cage, which is a frame fixed within a computer housing. The cage serves to position the circuit boards within the computer housing.
The various electrical components located on the backplane and cards (for example, the Dual In-Line Memory Modules (DIMMs) on the memory riser card) tend to consume a substantial amount of electrical power. In order to supply the electrical components with power, it is conventional to electrically connect the backplane to an external power supply. The backplane distributes the power received from the external power supply to the appropriate electrical components in a manner which will be subsequently described.
Backplanes are conventionally comprised of a plurality of superposed, laminated and alternating layers of conductive and insulative materials. The layers are each formed in discrete planes. For example, and referring to FIG. 1, a typical backplane 8 may have one or more internal wiring planes (i.e., a set of wirings located in one plane), each of which includes a number of individual conductive wirings 10 (also known as "traces"). The wirings are used to interconnect various electrical components and/or cards (not shown) locatable on the backplane 8 together, and allow the transmission of electrical signals.
Further, the backplane 8 will also typically include one or more power planes 12, which are typically sheets of conductive material, such as copper. The power planes are used to supply power from the backplane 8 to the various electrical components and/or cards located on the backplane 8.
Each of the conductive layers of backplane 8 are separated from the other conductive layers by a respective layer of insulating material. In FIG. 1, the power plane 12 is located beneath the plane of conductive wirings 10. Only a portion of the power plane 12 and conductive wirings 10 is shown for clarity of illustration.
Typically, the backplane 8 will be provided with a number of plated vias 14, 16 (i.e., holes plated or filled with a conductive material). Each via is electrically coupled to a respective internal power plane 12 or a respective wiring 10. The power is typically transmitted within the backplane 8, through the plating of the vias 14, through the respective internal power plane 12, and to the associated electrical components and/or cards. Likewise, the electrical signals are transmitted through the plating of the respective vias 16, through the respective wirings 10, and to an associated electrical component and/or card. As will be appreciated, to prevent shorting, it is important that the respective vias are only coupled to their associated corresponding wiring or power plane.
Further, in order to connect the backplane 8 to the external power supply, it is conventional to utilize a buss bar 18 (also known as a busbar or bus), such as is shown in FIG. 2. A buss bar is typically a heavy, rigid, metallic conductor, used to carry a large current and/or to make a common connection between several circuits. In the computer field, the buss bar is conventionally a relatively large, rectangular-shaped plate, composed of two conductive layers 20, 22 separated by a dielectric layer 24, for example. The conductive layers 20, 22 can be copper, with one of the conductive layers 20 being used to transmit power from an external power supply 25 to the backplane 8, and the other of the conductive layers 22 serving as a ground for the backplane. The advantages of a buss bar, as opposed to using a number of individual power transmission wires, each of which would be connected to the external power supply 25 and to a respective electrical connector, are well known to those skilled in the art. Such advantages include, for example, an organized power transmission scheme, due to the elimination of the power transmission wires, and a reduction in undesirable electrical noise, which may be generated when using a number of individual power transmission wires.
Typically, one edge of the buss bar 18 is provided with a plurality, for example, five sets, of conductive tabs 26, 26', each of which plugs into a corresponding electrical connector 28 that is connectable to the back plane 8. Various views of a conventional electrical connector 28 are shown in FIGS. 3-6. These electrical connectors 28 can be so-called solder tail power receptacles, (as illustrated) or C/P(compliant pin) power receptacles, for example, and will typically be provided with a plurality of pins 30. The pins 30 are arranged to be in registration with, and insertable into, a corresponding number of the plated vias 14 formed in the backplane 8.
Once the buss bar 18 is connected to the power supply 25 (in any conventional manner), and connected to the respective electrical connectors 28, the power will be transmitted through the conductive layer 20 of the buss bar 18, through the respective conductive tabs 26 of the buss bar, through the respective electrical connectors 28, and into the corresponding vias 14. Thereafter, the power is transmitted to the associated electrical components and/or cards in the manner previously described. To complete the circuit, the current travels back through a so-called ground plane, through the respective electrical connectors 28, through the respective conductive tabs 26' of the buss bar, and back to the negative terminal of the power supply 25, for example.
As will be appreciated, the use of power receptacles 28 requires that the associated vias 14 be positioned with predetermined spacings, so as to accommodate the pins 30 of the power receptacles. For example, if a power receptacle 28 has five pins 30 arranged in sequence, then there must be five vias 14 arranged in the same sequence, and with the same spacing, as the pins. Further, since the buss bar 18 will often have a plurality of sets of conductive tabs 26, 26', for example, five sets of conductive tabs, linearly arranged along its edge, then there will typically be a like-number of power receptacles 28 arranged in a similar manner, so as to receive the respective conductive tabs therein. This will require a proportional number of vias 14 to be prearranged in the backplane 8, to accommodate the respective pins 30 of the respective power receptacles 28.
This arrangement of the vias 14 may disadvantageously interfere with the placement of the various wirings 10 formed on or in the backplane 8. For example, if a via 14 is placed in the path of a wiring 10, then the wiring 10 may need to be diverted around the via 14 to ensure its continuity. Stated alternatively, the positioning of the tabs 26, 26' on the buss bar 18 will dictate where the vias 14 are located, and the positioning of the vias 14 will control the arrangement of the various wirings 10 on the backplane 8. However, this may entail additional costs in planning and designing the layout of the backplane 8, and require additional steps during the manufacturing of the backplane to ensure that the wirings 10 avoid the vias 14. Further, in order to avoid the placement of the vias 14, wirings 10 may be required to have added lengths with a resulting increase in resistance and decrease in performance. As will be appreciated, wirings having a short length are preferred in computer applications, due to the shorter distance required for electrical signals to travel. Moreover, since relatively low currents are transmitted to the associated circuitry through the wirings 10, the voltage drop due to the resistance of the wirings 10 preferably should be kept small. A low resistance will result in a relatively low voltage drop. Alternatively, if a via cannot be easily avoided, even by increasing a length of the wirings 10, it may be necessary to add further wiring planes, which likewise increases the cost (and thickness) of the backplane. Thus, there is a need for a means for transmitting the electrical current from a power source to a backplane that will not interfere with the placement of the wirings on the backplane.
Furthermore, this known arrangement requires that the power be channeled through the respective conductive tabs 26, 26', and into the respective power receptacles 28. However, the power receptacles 28 are limited in the amount of current that can be applied thereto. For example, conventional power receptacles have a 35-40 amp current limit. If more power is required, then a different system is needed for coupling the buss bar to the backplane. Thus, there is a need for a coupling arrangement that will electrically couple a buss bar to a backplane, which will allow for relatively high power transfers.
Additionally, the conventional connection between the buss bar and the backplane results in a relatively small number of so-called "A-spots". An A-spot is the pants of contact between adjoining surfaces. Because all surfaces are defined (on a microscopic level) by a plurality of peaks and valleys, when two such surfaces are placed in contact with each other, the two surfaces will only be in contact with each other at the highest of the peaks. Further, it is generally assumed that only one A-spot can be ensured between any two adjoining surfaces, i.e., at the point where the highest peak on either of the two surfaces contacts the other surface. As will be appreciated, this point of contact is typically quite small, with the size of the A-spot being dependent on the hardness of the two contact materials, and the contact force between the two contact materials. As the hardness of the materials increases, the size of the A-spot will decrease. Inversely, as the contact force between the two contact materials increases, the size of the A-spot will increase.
Since current can only flow from one conductive material to another adjacent conductive material at the A-spots, it is apparent that a larger A-spot will advantageously have a lower resistance, thus allowing the current to pass from one material to another more freely. Similarly, increasing the number of A-spots between two conductors will effectively reduce the resistance therebetween, thus also allowing the current to pass more freely.
The conventional buss bar 18, having five sets of conductive tabs 26, 26', will have a limited number of A-spots when plugged into the corresponding power receptacles 28. This severely limits the amount of current that can be transferred to the backplane 8. Further, although the size of each of the individual A-spots can be increased by increasing a contact force between the conductive tabs 26, 26' of the buss bar 18 and the respective power receptacles 28, this may result in further complications. For example, each conductive tab 26, 26' could be fastened to a corresponding power receptacle 28 using screws. However, this would make connecting the buss bar 18 to the backplane 8 more complicated, thus increasing assembly costs. Further, such arrangement would not increase the number of A-spots, only their respective sizes. Alternatively, and as shown in FIGS. 4-6, it is also known to provide each of the power receptacles 28 with a number of protruding contact bands 32, each of which contacts a respective conductive tab 26, 26' of the buss bar 18. While this will increase the number of A-spots per power receptacle, this will also result in a more complex, and hence more expensive, power receptacle. Further, the number of protruding contact bands 32 that can be located within a receptacle 28 is limited. For example, the illustrated receptacle 28 has twenty-two contact bands 32. As such, with this receptacle 28, there will be only twenty-two guaranteed A-spots per conductive tab. Moreover, there will be no A-spots in the locations between adjacent conductive tabs 26, 26'. Thus, there is need for an arrangement that will dramatically increase the number of A-spots at the connection between a buss bar and a back plane, without increasing the complexity of assembly.
Additionally, a substantial amount of force may be required in order to plug the conductive tabs 26, 26' of the buss bar 18 into the corresponding power receptacles 28, for a variety of reasons. For example, in order to provide for a larger A-spot, the protruding contact bands 32 within the respective power receptacles 28 will typically be adapted to exert a substantial amount of force (known as a "mating force") against the respective conductive tab 26, 26'. However, this arrangement will prevent the conductive tabs 26, 26' from being easily inserted into the respective power receptacles 28, since during insertion, the conductive tabs 26, 26' must urge the protruding contact bands 32 out of their respective positions. Moreover, if all of the respective power receptacles 28 are not precisely aligned with the conductive tabs 26, 26' of the buss bar 18, this misalignment will prevent the buss bar from being easily connected to the power receptacles. Thus, excessive force may sometimes be needed in order to plug the backplane 8 into the power receptacles 28. However, this may damage the power receptacle 28, the connection between the power receptacles and the backplane 8, and/or the backplane itself. Thus, there is a need for an electrical interface between a buss bar and a backplane that will allow the backplane to be electrically connected to the buss bar using a minimal amount of insertion force.
Further, power is conventionally supplied to buss bar 18 by connecting one end of the buss bar to the power supply 25. As previously noted, the current from the power supply 25 then passes through the buss bar 18, and to the respective power receptacles 28 by way of the conductive tabs 26. However, each conductive tab 26 is spaced away from where the current enters and exits the buss bar 18 by a different distance than the other conductive tabs. For example, the first conductive tab, i.e., the one located closest to the end of the buss bar 18 that is connected to the power supply 25, is located a first distance away from the end of the buss bar, the second conductive tab is located a second distance away from the end of the buss bar, and so on. As is apparent, the first distance is less than the second distance, the second distance is less than a third distance, and so on. However, since resistance is determined in part by the length of the conductor the current must flow through, the resistance to the current increases from the first conductive tab to the next conductive tab, i.e., the resistance at the first conductive tab is less than the resistance at the second conductive tab, and so on. Thus, there is a current attenuation from one conductive tab 26 to the next. Therefore, providing the conductive tabs furthest away from the power supply 25 with the necessary amount of current, may require that the current rating of the conductive tab nearest the power source be exceeded. This, of course, is undesirable. Therefore, there is need for a connector interface between a backplane and a buss bar that will allow current to enter the backplane in a uniform manner.